The present invention relates to an ink, and more particularly, to an aqueous ink for use in an ink ejection process.
German Auslegeschrift No. 2,164,614, hereby incorporated by reference, for example, discloses an ink ejection process in which temporary pressures are produced in an ejection head leading to the discharge of ink from capillary nozzles and to interruption of this ink flow to form droplets. To have an ink which is suitable for such ejection processes, certain parameters must be present. Thus, a clean printed picture requires, among other things, uniformly sized ink drops. These depend, on the one hand, on the geometry of the nozzle and on the configuration of the break-off plane. On the other hand, the ink must have as high a surface tension as possible. The supply of ink in the ejection head, for example, from an ink reservoir is often effected without pressure through capillaries. In order to attain a high ejection speed, however, the ink employed must have a low viscosity. The selection of suitable inks is limited, on the one hand, by its drying behavior. When the ejection system is at rest, ink components must not crystallize out in the area of the ejection head after long periods of inactivity. On the other hand, no surface film must form in the nozzle opening as a result of the evaporation of water. A slight increase in viscosity of the ink in the area of the nozzle, however, generally will not lead to malfunction of the ejection system, but, in the past, a character printed first after a period of inactivity will appear imprecise. The replenishment of solvents, such as water in this case, into the area of evaporation of the nozzle opening takes place through long capillary systems. In this case, similar physical properties are inherent in the ink in the nozzle exit area after longer periods of inactivity as they are encountered by ink drops without the addition of solvents under normal environmental conditions, such as the influence of temperature and humidity. Thus, the ink must remain fluid even with significant evaporation and must not deposit solids.
In order to avoid drying, moisturizing agents, such as glycerin, for example, are added to commercially available water-based inks. This additive, however, increases viscosity. The color intensity of such an ink is sufficient at most and its pH lies at 1.8 compared to a pH of 4 to 5 which would be favorable for ejection processes as defined above.